Such level limit switches are used in many branches of industry, especially in chemistry and in the food industry. They serve the purpose of limit state detection and are used for instance to secure against overfilling or to prevent pumps from running empty.
Depending on the application, special demands are made of these devices. For instances, if the predetermined level is very far away from a mounting opening for a suitable measuring instrument, then an extension, hereinafter called a tube extension, of the housing is provided, by which the mechanical oscillating structure can be placed at the site of the predetermined level and through which the electrical connection of the electromechanical transducer is accomplished.
In some applications, it is preferable to apply a coating to the device, for instance with enamel or a plastic such as polyfluoramide (PFA) or Halar (ECTFE). Naturally, the device must be capable of withstanding the very high temperatures that can occur when the coating is applied.
Applications also exist in which very high temperatures and/or pressures can act on the device.
In German Patent Disclosure DE-A 41 18 793, a device for determining and/or monitoring a predetermined level in a container is described, which device includes:
a mechanical oscillating structure to be mounted at the height of the predetermined level;
having a pot-shaped housing sealed on one end by a diaphragm, and
piezoelectric elements disposed in a stack,
which in operation serve to cause the mechanical oscillating structure to oscillate and to receive its oscillations that are dependent on an instantaneous level and make it accessible to further processing and/or evaluation.
The transducer has piezoelectric elements in the form of annular disks, arranged in a stack. One metal ring is disposed on each of the two ends of the stack. The metal ring toward the diaphragm rests on an annular shoulder, which is formed onto an outer annular surface of the diaphragm pointing into the housing. A clamping screw pointing to the interior of the housing is provided in the middle of the diaphragm. This screw is provided with an insulation and extends through the transducer. A nut is screwed onto the end of the clamping screw facing away from the diaphragm. This nut rests on the metal ring facing away from the diaphragm. The nut is tightened. The diaphragm is thus prestressed.